1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as an electrophotographic printer or an electrophotographic copying machine.
2. Description of the Related Art
In response to a recent increase in operating speed of a computer system, high speed and high reliability are demanded also in a printer as an output device for the computer system. In particular, an electrophotographic printer is expected as a printer meeting this demand. Initially, an optical fiber tube formed by integrating a flat cathode ray tube and an optical fiber array has been used as a light source for such an electrophotographic printer. The optical fiber tube has a merit such that a resolution thereof is high. However, it has defects such that small sizing of the light source is difficult and it is susceptible to an external magnetic flux. Further, a flying spot tube has also been proposed as a light source for a printer. However, since it has the same defects as those of the optical fiber tube, there are little cases utilized as a light source for a printer at present.
It is known that a laser beam printer and an LED array printer are widely used as an electrophotographic printer at present. In the laser beam printer, a laser beam is reflected by a polygonal mirror rotating at high speeds, and is scanned on a photosensitive body. Accordingly, high-speed printing can be effected. However, since the polygonal mirror is employed, an optical path is necessary to make it difficult to construct the printer as a whole in a small size.
In the LED array printer, a plurality of very small LEDs are arranged in array. Light emission from each LED is Controlled according to an image signal, and is applied directly or through a suitable optical system to a photosensitive body. The LED array printer has a defect that it is somewhat costly. However, since it has no mechanically movable parts, small sizing of the printer can be effected, and high speed can also be effected.
In an electrophotographic printer such as a laser beam printer or an LED array printer, a photosensitive drum is first uniformly charged with a given polarity. Then, the photosensitive drum is exposed to light according to image information to form an electrostatic latent image on the photosensitive drum. Then, the electrostatic latent image is developed by using a toner charged with the same polarity as that of the photosensitive drum. That is, the toner is electrostatically attached to an exposed portion of the photosensitive drum, thus forming a toner image on the photosensitive drum.
Then, the toner image on the photosensitive drum is transferred onto a sheet of paper normally by a transfer corotron in a transfer station. That is, the sheet of paper is guided to the toner image formed on the photosensitive drum, and DC corona discharge with a polarity reverse to that of the toner is normally applied by the transfer corotron to the paper from the back side thereof, thereby bringing the paper into close contact with the photosensitive drum and simultaneously electrostatically transferring the toner on the photosensitive drum to the paper. Then, AC corona discharge is applied by a separation corotron to the paper from the back side thereof, thereby separating the paper from the photosensitive drum. The paper thus separated from the photosensitive drum is fed to a fuser by a paper feeder, and the toner image on the paper is fused by the fuser, thereby obtaining a permanent image on the paper.
Recently, double-side printing in which permanent images are printed on both surfaces of the paper has been highly demanded. In performing such double-side printing, it is necessary to invert the paper in a paper inverting unit after fusing the toner image transferred to a first surface of the paper and then electrostatically attach the inverted paper to the photosensitive drum again. However, when the toner image formed on the first surface of the paper is fused by the fuser such as a flash fusing system employing a xenon lamp or the like, the paper is dried to be hardened and wrinkled. Accordingly, in electrostatically attaching the inverted paper having the permanent image on its first surface to the photosensitive drum by using the transfer corotron, a second surface of the paper is not brought into close contact with the photosensitive drum, causing a deterioration in transfer characteristic. As a result, a print quality is deteriorated.